This study will provide additional information about the physiology of the pulmonary capillary bed and is a continuation of a long term project to study the physiological and pathological mechanisms controlling the volume of blood in the pulmonary capillary bed as well as the distribution of that volume. The experimental approaches include: (1) Patients who had mitral stenosis and pulmonary hypertension and did not show a normal postural effect of tilting DL will be restudied several months after valvotomy to evaluate the effects of posture on DL and Vc; (2) The effects of preventing egress of blood from exercising limbs on DL will be evaluated in normal subjects during exercise with and without venous thigh cuffs; (3) Effects of exercise, Valsalva maneuver, Mueller maneuver, positive pressure breathing, negative pressure breathing, atropine administration, and body positions on gas distribution will be evaluated using DL determinations and radioactive Xenon distribution studies.